This invention relates to a method of producing magnesium vapor at atmospheric pressure.
Magnesium is produced industrially by several different processes. One of these processes is the so-called Magnetherm Process described in U.S. Pat. No. 2,971,833. The Magnetherm Process involves a reaction between a metallic reducing agent and magnesium-oxide in the presence of a liquid mixture of oxides in a reaction zone which is heated by the electrical resistance of the mixture of oxides. In carrying out this process, a magnesium oxide ore, such as calcined dolomite, and a reducing agent comprised of silicon, ferrosilicon or an alloy of aluminum and ferrosilicon are charged to the reaction zone of a reaction-condensation system. Aluminum oxide is also added to the reaction zone and the composition of the total charge is controlled so that a particular liquid slag mixture of calcium, silicon, aluminum and magnesium oxides, is formed and maintained in the reaction zone. Existing Magnetherm Processes are typically operated at 1550.degree. C. and at low pressures, such as 70 torr (0.09 atmospheres). A vacuum must be used in order to remove the magnesium vapor from the vessel to a condenser, where the vapor is condensed in order to form magnesium metal. This low pressure operation often leads to low magnesium recovery due to oxidation of the liquid and gaseous magnesium wherever an air leak occurs.
There have been disclosed Magnetherm Processes which operate at atmospheric pressure (or 760 torr). U.S. Pat. No. 5,090,996, the disclosure of which is expressly incorporated herein by reference, discloses using a plasma arc to heat the molten slag in a reaction zone. The plasma arc can generate much higher temperatures (on the order of 1800.degree.-2000.degree. C.) at the reaction zone than conventional electrodes. The patent states that the slag composition should be controlled so that it is wholly within the periclase region of the slag composition phase diagram depicted at FIGS. 1-6 therein, with a substantially constant liquidus temperature of about 1800.degree.-2000.degree. C. and preferably 1900.degree.-1950.degree. C. The patent further states that controlling the molten slag composition and using high temperatures produces magnesium vapor at atmospheric pressure.
Although U.S. Pat. No. 5,090,996 discloses a slag composition that can produce magnesium vapor at atmospheric pressure, there are several shortcomings of the disclosed process that may prevent full and efficient utilization thereof. First, the temperature at the surface of the reaction zone must be maintained at a very high 1900.degree.-1950.degree. C. in order to insure that the slag composition is wholly within the periclase region and wholly within one phase, the liquid phase of said diagram. Current Magnetherm Processes operate at much lower temperatures, around 1550.degree. C. There are several negative consequences of operating at temperatures in excess of 1900.degree. C. including the fact that energy costs to generate such higher temperatures will be increased significantly. The high temperatures will also create a more "aggressive" molten slag that may attack the refractory lining of the furnace, leading to shorter life of the refractory and the need for more frequent replacement thereof and thus more furnace "down time". Finally, undesired reactions, such as the production of carbon monoxide, can take place at these higher temperatures.
Another shortcoming of the process disclosed in U.S. Pat. No. 5,090,996 is that the slag composition must have a higher magnesium-oxide content than slags used in the existing Magnetherm Processes. At column 9, the patent discloses slag compositions having on the average 20%-25% magnesium-oxide. This means that more magnesium-oxide containing feed materials must be used to produce a given amount of magnesium vapor. Obviously, this fact negatively impacts on recovery rates and overall economic viability of the process.
Yet another shortcoming of the process disclosed in U.S. Pat. No. 5,090,996 is that the slag composition must be controlled by alternate feeding of the metal reductants, Al and Si (or FeSi). As is known to those skilled in the art, feed rates and feed amounts must be precisely controlled and it is easier to control a steady feed rate and amount than it is to precisely control alternating feed rates and amounts of metal reductants. Furthermore, the particular reductants alternated in U.S. Pat. No. 5,090,996 create a changing slag composition that can aggressively attack the refractory lining of the furnace leading to still other problems discussed above.
Therefore, there remains a need for a method of producing magnesium vapor at atmospheric pressure that overcomes the shortcomings of the prior art.